Collaborative Research: Testing the Role of Magma and Related Fluids in Early-Stage Rifting, East Africa

该项目的目标是以坦桑尼亚北部和肯尼亚南部的东非裂谷系统的一部分为目标，制定一种综合的方法来了解在厚厚的大陆岩石圈中形成的早期和年轻的裂谷地带。这些裂谷的发展是一个基本的板块构造过程，但裂谷形成和早期演化背后的驱动力仍被细致入微地理解，特别是岩浆作用和构造作用(岩脉侵入与断裂作用)的应变调节之间的反馈和权衡。此外，在裂谷系统的发展中，热液流体和先前存在的结构在削弱岩石圈和集中变形和岩浆活动方面的综合作用几乎没有被探索过。该项目旨在解释早期裂谷下岩浆系统的相对作用，大型边缘断裂和跨裂谷分布的正断层系统的发育，以及热液对大陆岩石圈的削弱和促进裂谷集中带的发育和演化的地球化学影响。这种分析将利用：(1)现场绘制断层系统几何和动力学图，以描述与岩浆系统有关的应变的空间模式；(2)利用地球物理成像(宽带地震阵列)，三维可视化岩石圈中的岩浆和断层系统，以及沿裂谷带段长度的地壳和上地幔结构；(3)沿边界断层系统对热泉的气体排放进行采样，以确定可能有助于岩石圈内减弱的流体的化学特征；以及(4)对断层系统暴露的火山产物进行采样，以进行地质年代学分析，提供与岩浆幕有关的断层事件的时间约束，并允许量化时间平均应变率。这项研究将建立裂谷早期阶段岩浆和断层系统相互作用的综合模型，作为评估全球大陆裂谷演化的基准。大陆裂谷带之所以神秘，是因为它们发育在厚实的大陆岩石圈中，原因尚不清楚。它们的发展是板块构造过程的关键，然而，它们促进了大陆的分裂，通过大洋中脊的扩张过程形成了洋盆。世界各地的裂谷地带通常与大地震和火山活动有关，这些活动要么集中在离散的火山上，要么从数十公里长的裂缝中喷发。因此，它们构成了威胁生活在包括美国在内的世界各地裂谷带环境中的人类的地质灾害。过去关于裂谷带发展后期阶段(大陆解体之前)的工作表明，大陆岩石圈因裂谷作用而逐渐伸展和减薄，从而使地幔上升到裂谷之下并融化。然而，在年轻的和发展中的裂谷系统中，岩浆似乎对裂谷作用很重要，尽管在那个阶段地幔上升很少。这表明，尽管存在厚厚的大陆岩石圈，但岩浆能够形成并上升到地壳中，使之能够伸展，并帮助驱动断层。岩石圈显然在某种程度上被削弱了，从而使岩浆活动如此集中。这个研究小组假设，岩石圈的减弱得到了与岩浆系统发展有关的集中的深部地幔流体的帮助，这些流体在化学上改变和削弱了岩石圈，允许岩浆侵入并驱动裂谷的发展。在东非裂谷的这项工作将使研究人员能够确定这些流体和侵入的岩浆在削弱岩石圈和允许断层发育方面的相对作用。通过将野外工作、地球物理、地球化学和地质年代学相结合，将形成一个跨学科整合的裂谷发展和演化模型，并将捕捉裂谷形成过程的多个方面。这项工作的结果将适用于全球裂谷地带的发展，因此可以整合到现有的板块构造模型中，为为什么大陆分裂是可能的提供了理论基础。

The goal of the project is to develop an integrative approach to understanding incipient and youthful rift zones developing in thick continental lithosphere by targeting a portion of the East African Rift system in northern Tanzania and southern Kenya. The development of these rifts is a fundamental plate tectonic process yet the driving forces behind rift initiation and early evolution are tenuously understood, particularly the feedbacks and trade-offs between strain accommodation by magmatic and tectonic processes (dike intrusion vs. faulting). Furthermore, the integrative roles of hydrothermal fluids and preexisting structure in weakening the lithosphere and focusing deformation and magmatic activity are virtually unexplored in developing rift systems. This project seeks to explain the relative roles of magma systems below incipient rifts, the development of large border faults and systems of distributed normal faults across rift valleys, and the geochemical influences of hydrothermal fluids in weakening thick continental lithosphere and promoting the development and evolution of focused rift zones. This analysis will utilize: (1) field-based mapping of fault system geometries and dynamics to characterize spatial patterns of strain in association with magmatic systems; (2) 3D visualization of magmatic and fault systems in the lithosphere, and crustal and upper mantle structure along the length of the rift zone segments, using geophysical imaging (a broadband seismic array); (3) sampling of gas emissions from hydrothermal springs along border fault systems to determine chemical signatures of fluids that may contribute to weakening within the lithosphere; and (4) sampling of volcanic products exposed by fault systems for geochronologic analysis, providing temporal constraints on faulting events in relation to magmatic episodes and allowing a quantification of time-averaged strain rates. This study will result in the development of an integrated model of magmatic and fault system interactions during the early stages of rifting that can serve as a benchmark for evaluating continental rift evolution globally.Continental rift zones are enigmatic in that they develop in thick, strong continental lithosphere for reasons that are poorly understood. Their development is a linchpin of the of plate tectonic process, however, facilitating continents breaking apart, forming ocean basins by the process of mid-ocean ridge spreading. Rift zones worldwide are commonly associated with large magnitude earthquakes as well as volcanic activity, either focused at discrete volcanoes or erupting from tens of kilometer long fissures. They thus present geologic hazards that threaten humans living in rift zone environments worldwide, including the U.S.A. Past work on rift zones in advanced stages of development (prior to continental breakup) has shown that the progressive stretching and thinning of the continental lithosphere by rifting allows the mantle to well up beneath the rift and melt. In young and developing rift systems, however, magma appears to be important for rifting despite very little mantle upwelling at that stage. This suggests that, despite the presence of thick continental lithosphere, magma is able to form and ascend into the crust, enabling extension and helping to drive the faulting. The lithosphere is clearly weakened in some manner to allow magmatic activity to be so focused. This research team hypothesizes that lithosphere weakening is assisted by focused, deep mantle fluids associated with developing magmatic systems and that these fluids chemically alter and weaken the lithosphere, allowing magma to intrude and drive rift development. This work in the East African Rift will allow researchers to determine the relative roles of these fluids and intruding magma in weakening the lithosphere and allowing faults to develop. By combining fieldwork, geophysics, geochemistry, and geochronology, a model for rift development and evolution will be formulated that is integrated across disciplines and will capture multiple facets of the rifting processes. The results of the work will be applicable to rift zone development globally and can thus be integrated into existing plate tectonic models to provide a rationale for why continental breakup is possible.